A combustion turbine is an engine that typically comprises a compressor to draw in and compress a gas (usually air), a combustor to add energy to the compressed gas, and a turbine driven by the resulting expansion of heated gas. The turbine, in turn, can be used to power a wide range of equipment including ships, aircraft, and power generators.
The turbine typically comprises one or more stages of blade assemblies extending from a rotatable shaft and stationary guide vanes usually located adjacent the combustor. The heated gas from the combustor, guided by the guide vanes, impinges upon the blade assemblies to drive the turbine.
Both the blade assemblies and guide vanes typically comprise airfoils. An airfoil of a blade assembly usually extends outwardly from a platform connected to a root. The root, in turn, is mounted to a turbine disk on the shaft. The hot gases from the combustor flow over the airfoils during operation of the combustion turbine causing the airfoils to rotate.
Because of the high temperatures of the gases, it may be desirable to cool the airfoils. U.S. Pat. No. 5,688,104 to Beabout, for example, discloses multiple cooling passageways that extend from a platform through an airfoil to carry cooling gas therein to a cooling or diffusion hole in an outer wall of the airfoil. Each diffusion hole comprises a metering section and a diffusing section, the later terminating at an opening in the surface of the airfoil wall. Each opening expands symmetrically in a radially outward and a radially inward direction to diffuse cool air from a corresponding passageway into the path of the hot gases carried by a gas flow in a downstream gas flow direction. The cooling gas provides a cooling film over the surface of the airfoil.
U.S. Pat. No. 6,368,060 to Fehrenbach et al. similarly discloses cooling passageways that terminate at corresponding diffusion holes that also have symmetrical expansions. In addition, Fehrenbach et al. discloses forming cooling holes with a high pressure fluid stream and coating the airfoil surface with a ceramic layer to assist in airfoil cooling.
Rotation of the airfoil of a blade assembly exerts a centrifugal force on the cooling gas molecules. Thus, the cooling gas may not properly diffuse as it exits a conventional, symmetrical diffusion hole. Reorienting a conventional, symmetrical diffusing hole, however, may result in an over expansion problem in the sense that the surface outlet of the cooling hole may open too much in a radially outward direction. The result may be separation of the hot gases carried in the downstream gas flow direction and their entry into the diffusion hole.
Similarly, if the diffusion hole expands in the upstream gas flow direction, the cooling gas may meet the hot gas at a perpendicular or more nearly perpendicular angle. This may result in further diminution of the cooling effect intended. It may also result in hot gas entering the diffusion hole. Accordingly, with conventional diffusion holes, one may be constrained to accept a less desirable orientation of the gas flow or risk causing an over expansion problem.